Technical Abstract:
The Early Soybean Production System (ESPS) has been applied to the Southern Central area in the U.S. to maintain soybean (Glycine max Merrill) yield by avoiding drought. However, ESPS can produce seeds with good compositional quality but low germination and vigor. Our hypothesis is that high temperature stress during mid- to late-seed production (R5-R8) in the ESPS can account for the low germination and vigor of seed. This research was designed to study the effects of heat stress on soybean seed quality (seed composition, germination and vigor) and to explore the potential mechanisms of heat impairment to seed vigor. Plants of soybean breeding line N98-4445A were grown in growth chambers at 27/18ºC day and night temperature with 13.5 h photoperiod. Seed development was divided into 10 stages starting at R5. Embryos acquire the ability for germination at stage R5-6 and for desiccation tolerance at stage R5-5. Therefore, plants were heat-stressed (by 37/30ºC) at R-stages prior to achieving germinability (between R5.4 and R5.5) or during seed maturation, R7 for 1 d, 4 d or until harvest, respectively. Seeds were analyzed for seed composition, germination, and seedling vigor. It has been shown that heat treatment at an early stage (R5) for longer time periods (from R5 until harvest) impairs the seed more severely than other treatments. The electrical conductivity of the water after soaking seeds was significantly associated to seed vigor, indicating that the heat stress treatments damaged the cell membrane integrity of the seed. Leakage of valuable nutrients can reduce and impair seedling growth and development. Investigations on seed composition and proteomic changes in response to heat stress are ongoing.